As is well known, elevator cars suspended in a hoistway are guided by rails which are typically positioned to guide the sides of the elevator car.
The installation of rails in a perfectly straight and parallel fashion is a difficult task. Additionally, settlement and other adjustments in building dimensions cause rails to require periodic straightening so as to ensure smooth, parallel tracks upon which the elevator can ride smoothly. In order to measure the position of the rails along an entire hoistway, it is known to use a taut wire extending vertically near the rail, and measuring the distance between the wire and the rail in both the side-to-side (rail-to-rail) and front-to-back directions. This requires gaging the distance at a large number of points and is extremely time-consuming. It is also known to measure the distance between the rail and an elevator or a cross-rail gage or spacing bar. Such a measurement can be achieved with gages, or with electromechanical position sensors, such as linear variable differential transformers (LVDTs), hall effect devices and the like. Elevator rail gages (such as LVDTs) which contact a taut wire have the disadvantage of inducing oscillations in the wire, which prevents use of such gages other than at extremely low speeds, while standing still, or with rest periods between movement and the recording of measurements.
A more recent approach to handling the rail alignment problem in elevators includes active compensation, which positions the elevator car different distances from the rail so as to keep the elevator car more nearly within a single pair of vertical planes as it travels vertically in the hoistway, In this approach, a map of rail positions is made, as a function of the vertical position in the hoistway, and this map is used to force the elevator car to be the correct distance from the rail with electromechanical actuators. Providing the map of rail positions can be achieved in any of the known ways referred to hereinbefore, but is currently being achieved by means of acceleration measurements taken with accelerometers while the elevator car is moving within the hoistway. As is known, acceleration can be doubly integrated to provide position, which in turn can create the map of compensation to be utilized. An example is shown in a commonly owned, co-pending U.S. patent application entitled "Measurement of the Horizontal Deviations of an Elevator Car Vertical Hoistway Rail" Ser. No 07/668,544 filed on Mar. 13, 1991 by Roberts. However, determining position with accelerometers has the disadvantages of a time lag, which requires correlating the data to proper position, and of false readings due to forces within the elevator car other than those imposed by the rails. An elevator active compensation system which purports to work directly from instantaneously sensed deviations from a taut wire (plumb line) uses gap sensors to determine the relative position of the elevator car with respect to the taut wire, is shown in Ando U.S. Pat. No. 4,754,849. However, the measuring of distance by means of a gap sensor is tenuous at best, and with respect to a taut wire, is very difficult. The issues of sensitivity, linearity and cross coupling are not addressed in the Ando patent. A detector using taut wires having optical wire sensors on either ends of a spacing bar is shown in Japanese Patent 232680, Oct. 16, 1991. In Sikora Patent 4,088,952, a system for providing a visual indication of the deviation of a wire from a nominal position employs a pair of coils, on opposite sides of the wire, which respond to AC current in the wire. Since this is a nulling system, it is not concerned with linearity; since only one axis is involved, it does not deal with cross-axis coupling at all.
Another innovation has been the use of lasers to align rails. However, these are limited to use in buildings having only a few tens of floors, and are complex and expensive.